The present invention relates to methods and apparatus for providing a non-oxidizing atmosphere at the surface of a wave soldering bath to discourage the formation of oxides on the liquid solder surfaces.
Wave soldering machines have been introduced for a long time in the industry to automatically solder components on a printed circuit board which operation was previously done by hand. A usual wave soldering machine comprises at least one preheating zone to preheat the printed circuit board, at least one soldering zone to solder the components to the board by coating the bottom side of the printed circuit board with molten solder contained in a solder pot, and at least one cooling zone where the solder is solidified. This soldering process, or coating process, is usually conducted in the presence of a fluxing agent which agent is used to improve the wetting of the copper surface on the bottom of the printed circuit board which surface needs to be joined or coated. The fluxing agents are usually corrosive and the excess or residue of these agents must be cleaned after the wave soldering operation.
Low residue no-clean fluxes or flux-less processes have been developed wherein it is possible to carry out the wave soldering process without the inconvenience of standard fluxing agents, under a substantially oxygen-free atmosphere, such as nitrogen.
U.S. Pat. No. 3,705,457 discloses one of the earliest wave soldering processes, including injection of an inert gas to avoid oxidation of the metallic surfaces of the printed circuit board.
U.S. Pat. No. 4,538,757 discloses a wave soldering process under a reducing atmosphere comprising nitrogen and hydrogen, and nitrogen curtains at entrance and exit of the machine to inhibit atmosphere exchange with the ambient air.
U.S. Pat. No. 4,606,493 discloses a method and apparatus for soldering printed circuit boards under an inert gas atmosphere to prevent oxidation of the electrical (usually copper) connections due to the heat produced during soldering and reduce the occurrence of thermal stress defects in the circuit carrier. To this end, an inert gas is injected through slits to provide a plurality of jets of high velocity which impinge the bottom side of the printed circuit board. As a condition of operation, the temperature of the inert gas jets is about twice as high as the temperature of the molten solder in the solder pot (600.degree. C.).
U.S. Pat. No. 4,646,958 discloses a solder reflow, or solder chip process which is carried out in a flux-less or flux free system, under an atmosphere comprising nitrogen and saline, or hydrogen and saline.
U.S. Pat. No. 4,821,947 discloses a process to coat a molten metal to a metal-comprising surface without using a flux. This process is carried out in an inert atmosphere in which the temperature is sufficiently low that no damage is done to the metal-comprising surface, and no damage is done to materials such as components adjacent to the metal-comprising surface.
U.S. Pat. No. 5,071,058 discloses a process for conducting a joining/coating operation which is carried out in a controlled oxidizing atmosphere, having an oxidation capability greater than that required to oxidize a metal-comprising filler material used for joining or coating, but having less oxidation capability than that of air. In case of a wave soldering process the oxygen content in the inert gas atmosphere is at least 10 ppm and preferably at least 500 ppm.
U.S. Pat. No. 5,121,875 discloses a short hood for wave soldering machines, wherein preheating of the printed circuit boards is carried out under air. In this process a no-clean flux is used and an oxygen concentration which is less than 5% is recommended at the solder pot.
U.S. Pat. No. 4,921,156 discloses an apparatus having a soldering chamber and comprising means to inject a protective gaseous atmosphere in the soldering chamber and sealing skirt means protruding downwardly into the pool of molten metal solder. Preferably the protective gaseous atmosphere is comprised of nitrogen and possibly of some reducing agent.
U.S. Pat. No. 4,746,289 discloses a process for treating parts under a non-reactive atmosphere with laminar flow conditions. The laminar flow conditions disclosed in this patent usually apply for inert gas injection in wave soldering machines.
In sum, a substantially oxygen-free atmosphere has been achieved by a so-called fully inerted wave soldering system, and a dross reduction boundary system.
The fully inerted wave soldering systems currently uses a tunnel type of system. That type of system is very expensive and time-consuming to install. Because this type of machine uses a tunnel, the access to the assemblies being soldered is greatly reduced. To achieve the desired results that type of system must operate at very high gas flow rates (over 1500 scfh). By doing this, the oxygen ppm level in the inerting system is kept low, thus yielding the desired results. If the flow rate is reduced, the atmosphere becomes unstable and benefits are lost. The primary goal of the fully inerted system is keep the oxygen ppm level below approximately 100 ppm. This then would yield the maximum dross reduction with the greater wettability for soldering.
The dross reduction boundary inerting system was developed to address the problems of the fully inerted system. The design was such that it can be easily installed and is much lower in cost. The other goal of this system was to have a reduced inerting gas flow rate. While those goals appear to have been met, the performance of that system is greatly reduced as compared to that of the fully inerted system. Because the boundary inerting system depends on a circuit board being present for the inerting to take place, the actual inerting effect is never fully achieved. The dross reduction of such a system is typically at best 70% (with reference to a solder pot operating without an inerting gas) with only marginal if any improvements in wettability.
Examples of a dross reduction boundary inerting system can be found in U.S. Pat. Nos. 5,203,489 and 5,240,169. There is disclosed therein a system in which solder waves are established by wave pumps in a solder reservoir. A conveyor transports circuit boards so that their undersides pass through the solder waves. As depicted in FIG. 10, a cover plate C extends across the top of the reservoir R and includes recesses to accommodate the solder waves. On the sides of each solder wave there is thus formed a chamber CH bordered on its top by the cover, on its bottom by the solder reservoir, one side by the solder wave, and on an opposite side by either (i) a stationary wall (of the solder pot or solder pot housing), or (ii) another solder wave (in the case where the chamber is situated between two solder waves). Each chamber includes a gas outlet defined by a gap G formed between the solder wave SW and an edge of the cover slot.
Disposed within each chamber is a gas discharge pipe P extending parallel to the length of the solder wave. A shield gas is discharged from orifices in the gas discharge pipe to create an inert atmosphere within the chamber CH. A flow F of shield gas exits the chamber through the outlet. That gas flow is intended to provide an inert blanket across the portion of the wave side which projects above the gap G when a circuit board is present. However, when no circuit board is present, the portion TS of the top surface of the wave which curves away from the gap G is exposed to any ambient oxygen which may be present in the ambient atmosphere, whereupon dross can be formed. In the case of soldering operations in which there occurs a wide spacial gap between successive circuit boards being conveyed to the solder pot, it is not unusual for the solder wave pumps to be deactivated between the soldering of successive boards in order to minimize dross formation.
Furthermore, the behavior of the gas flow F emerging from the chamber can actually promote the formation of dross on the solder wave. In that regard, the pressure difference between the gas in the discharge pipe on the one hand and the atmosphere disposed above the chamber outlet, on the other hand, is relatively low. As a result, the velocity of the gas traveling within the chamber toward the outlet is relatively slow. Due to its slow travel, the gas will be significantly heated by the hot solder wave. Consequently, the hot gas flow F emerging from the gap G has a tendency to rapidly rise and create a free swirling convection current CC which draws the cooler atmosphere (and any oxygen contained therein) downwardly toward the solder wave, thus resulting in the formation of dross.
Another region in which dross is formed in the solder is at the place where the drive shafts for the wave pumps enter the solder reservoir. The rotation of those shafts produces a churning of the solder, whereby oxidation is promoted.
It will be appreciated that the dross formed in the solder eventually builds up to a level requiring that the solder pot be shut-down to enable the dross to be skimmed off the top of the solder reservoir. The frequency at which those costly shut-downs occur is a function of the rate of dross formation.
Therefore, it would be desirable to minimize the rate of dross formation beyond the rates currently achieved.